Coating method utilizing two coating materials

ABSTRACT

A COATING METHOD FOR CONTROLLING THE INDEX OF REFRACTION OF A LAYER UPON A SUBSTRATE BY UTILIZING TWO COATING MATERIALS HAVING DIFFERENT INDICES OF REFRACTION BY EVAPORATING EACH OF THE COATING MATERIALS TO PROVIDE A VAPOR STREAM WHICH IMPINGES UPON THE SUBSTRATE AND MASKING THE SUBSTRATES FROM THE VAPOR STREAM TO PROVIDE A VAPOR STREAM CARRYING THE COATING MATERIALS TO THE SUBSTRATE IN A PROPORTION RELATED TO THE DESIRED INDEX OF REFRACTION.

A. J. THELEN ET AL March 26, 1974 COATING METHOD UTILIZING TWO COATING MATERIALS Original Filed March 14. 1966 5 Sheets-Sheet 1 km m NN ME March 26, 1974 THELEN ETAL 3,799,800

COATING METHOD UTILIZING TWO COATING MATERIALS Original Filed March 14. 1966 5 Sheets-Sheet z March 26, 1974 A, J THELEN ETAL 3,799,800

COATING METHOD UTILIZING TWO COATING MATERIALS 5 Sheets-Sheet 3 Original Filed March 14. 1966 March 26, 1974 E ETAL' 3,799,800

COATING METHOD UTILIZING TWO COATING MATERIALS Original Filed Harch-l4. 1966 5 Sheets-Sheet 4 March 26, 1974 J THELEN ETAL 3,799,800

COATING METHOD UTILIZING TWO COATING MATERIALS Original Filed March 14. 1966 5 Sheets-Sheet 5 203 v 9/ 202 SCHMITT 207 208 STEPPING v TRIGGER MOTOR K I I BISTABLE SHAFT DIFFER- FLIP POWER T POS. 9 FLOP MR 9 POT. F. 1 I

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k/ NON LINEAR AMP. 219 22! 222 223 l v 2 DOUBLE DIFF COMPENSATOR COMPENSATOR SCHMITT TRIGGER a RELAY R6, +1) R: (smcam K0 R, ($R,c1 +1) R. (smc, +1)

MANUALLY CONTROLLED 229K VARIAC 5 I 226 c BOAT a I MOTOR STEP TRANS. 27 DRIVEN oowu T VARIAC TRANS. 'L-i |i $75 I 28 2I8 K .1 K6 36 s Rm; 2,

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United States Patent 3,799,800 COATING METHOD UTILIZING TWO COATING MATERIALS Alfred J. Thelen, Nils H. Bergfelgand Eugene A. Eufusia,

Santa Rosa, Calif., assignors to Optical Coating Lahoratory, Inc., Santa Rosa, Calif.

Original application Mar. 14, 1966, Ser. No. 533,996, now Patent No. 3,636,916. Divided and this application July 19, 1971, Ser. No. 164,130

Int. Cl. C23c 11/00 US. Cl. 117-106 R 3 Claims ABSTRACT THE DISCLOSURE A coating method for controlling the index of refraction of a layer upon a substrate by utilizing two coating materials having different indices of refraction by evaporating each of the coating materials to provide a vapor stream which impinges upon the substrate and masking the substrates from the vapor stream to provide a vapor stream carrying the coating materials to the substrate in a proportion related to the desired index of refraction.

This is a division of application Ser. No. 533,996, filed Mar. 14, 1966, now Pat. No. 3,636,916.

In coating operations, it often is desirable to utilize two coating materials and to deposit them simultaneously in a predetermined relationship. With existing coating machines, this is very diflicult to do. In addition, it is very dilficult to monitor the rate of deposition of each type of material. There is, therefore, a need for a new and improved coating apparatus, system and method, and a rate monitor for use therein.

In general, it is an object of the present invention to provide a method in which one or more evaporation sources may be utilized independently of each other or at the same time.

Another object of the invention is to provide a method of the above character in which the evaporation rate from any source can be controlled automatically.

Another object of the invention is to provide a method of the above character in which the rate of evaporation from the coating sources can be made to follow a preprogrammed function.

Another object of the invention is to provide a method of the above character in which the proportions of the diiferent coating materials can be readily controlled.

Another object of the invention is to provide a method of the above character in which the proportioning of the coating materials is automatically programmed.

Another object of the invention is to provide a method of the above character in which the rates of deposition of the materials can be monitored optically.

Another object of the invention is to provide a method of the above characterwhich does not require'the use of separate chips. 7

Another object of the invention is to provide a method of the above character. which is particularly useful for controlling deposition where a constant rate of deposition is required.

Additional objects and features of the invention will appear from. the following description in which the preice ferred embodiment is set forth in detail in conjunction with the accompanying drawings.

Referring to the drawings:

FIG. 1' is a cross-sectional view of a coating incorporating the present invention.

FIG. 2 is an enlarged cross-sectional view ofthe mask indexing assembly.

FIG. 3 is a view looking along the line 3-3 of FIG. 2 and showing the masks of the mask indexing assembly.

FIG. 4 is an enlarged cross-sectional view of the rate monitor.

FIG. 5 is a view looking along the line 55 of FIG. 4 and shows the cover plate with the aperture therein and the mask for adjusting-the size of the aperture.

FIG. 6 is a block diagram of the electrical circuitry for the mask indexing assembly.

FIG. 7 is a block diagram of the electrical circuitry for the automatic rate control system.

In general, the coating apparatus and system includes a chamber with means mounted in the chamber for carrying a plurality of substrates to be coated. First and second sources for vapor streams are disposed within the chamber. A particularly unique means is provided for monitoring the rate of evaporation from the sources. In addition, means is connected to the monitoring means and to the sources for causing the evaporation rate from the vapor sources to follow a pre-programmed function. In addition, masking means is provided for controlling the flow of vapor from each of the sources to the substrates being coated. Means is provided for automatically controlling the masking assembly to cause the deposition of the coating materials upon the substrate in a predetermined relationship.

As shown in the drawings, the coating apparatus incorporated in the present invention consists of an enclosed vacuum chamber or housing 11 which includes a side wall 12, a bottom wall 13 and a top wall 14. First and second source assemblies 16 and 17 are mounted in the bottom wall and are adapted to supply vapor streams to the substrates 18 to be coated carried by a rotatable rack assembly 19. A mask indexing assembly 21 is mounted in the bottom wall between the first and second source assemblies 16 and 17. Means is provided for monitoring the rate of deposition of vapors upon the substrate 18 and consists of first and second substrate shifting assemblies 22 and 23 and first and second optical monitors 26 and 27. The optical monitors are of a type disclosed in copending application Ser. No. 321,888, filed Nov. 6, 1963, now Pat. No. 3,411,852, and as disclosed therein, each includes a sensing assembly 28.

The first and second source assemblies 16 and 17 are substantially conventional and each consists of a pair of conducting rods 29 and 31 carried by'mounting plate 32 formed of insulating material secured to the bottom Wall 13. The rods 29 and 31 extend through an opening 33 provided in the bottom wall 13. The rods 29 and 31 carry a support assembly 34 which mounts a boat 36. As is well known to those skilled in the art, thesupport assembly 34 is arranged so that when power is supplied to the rods 29 and 31, power flows through the boat 36 which is formed of a resistive type material to heat the source material carried therein to cause the same to apparatus Patented Mar. 26, 1 974 vaporize within the chamber 11. Power are connected to a suitable source of power (not shown).

As explained previously, the vapors from the boats 36 are adapted to impinge upon the substrates 18 carried by the rack assembly 19. The rack assembly 19 consists of a rack 41 of conventional construction which is mounted upon a spindle 42 rotatably carried by ball bearing assemblies 43 mounted in a bearing housing 44. The spindle 42 is also held in place by a plate 46 which is secured to the lower extremity of the bearing housing 44 by screws 47. The bearing housing 44 extends upwardly through an opening 48 provided in the top wall 14 and is secured to a mounting plate 49 which is secured to the top wall 14 by cap screws 51. A shaft 52 is rotatably mounted in the mounting plate 49 and is connected to the spindle 42. by a pin.53. The shaft 52 is rotated by suitable motive means such as an electric motor and a speed reducer (not shown) to rotate the rack 41 at a predetermined rate.

MASK INDEXING ASSEMBLY 21' The flow of vapors to the substrates 18 carried by the rack 41 is also controlled by the mask indexing assembly 21. The maskindexing assembly 21 is shown in detail in FIG. 2 and consists of a large housing 61 which is secured to a mounting plate 62 by screws 63. The mounting plate 62 overlies an opening 64 provided in the bottom wall 13. Suitable means is provided for forming a seal between the plate '62, the bottom wall 13 and the housing 61 and eonsists of O-rings 66 and 67.

The mask indexing assembly 21 also includes first and second masks 71 and 72 which are carried by concentric shafts 73 and 74. The mask 71 is a 180 mask as hereinafter described and is secured to the shaft 73 by a split hub 76. The mask 72 consists of two 90 masks and is mounted upon a split hub 77 which is mounted upon an enlarged end member 78 secured to the upper extremity of the shaft 74 by suitable means such as welding. The shaft 74 is rotatably mounted in ball bearing assemblies 79 carried in a bearing housing 81. The bearing housing 81 is provided with a flange 82 which is secured to thehousing 61 by screws 83. The bearing assembly 79 is heldin place by retaining rings 84. The bearing housing 81 is pro.- vided with openings 86 and the shaft 74 is provided ,with openings 87 which communicate with the interior of ,the chamber 11. The inner concentric shaft 73.,is rotatably mounted within the outer shaft 74 by a ball bearing as,- sembly 88 which is held in place by a retaining ring 89.

Means is provided for driving the shafts 73 and 74 and consists of gear motors 91 and 92. The gears motors91 and 92 can'be of any suitable type as, for example,.,the gear motor 91 can have a range of output speeds from zeroto 100 r.p.m., whereas the gear motor 92 can have a range of output speeds from zero to 60 r.p.m. The gear motors 91 and 92 are mounted upon a plate93 whichis secured to the housing 61 by screws 94. A suitable .seal is provided in the form of an O-ring 96.

The gear motor 92 is provided with an output shaft 97 which is connected to a coupling 98 by the tongue and slot connection shown. The coupling 98 is connected to the center shaft 73 by a pin 99. The coupling 98 isrosupplied to the source assemblies 16' and 17 byt'ra'n'sformers 37 which retaining rings 116. A large planetary gear 117 is mounted on'the'larg'e hub 113 and is driven by the sniall' g'ear'118 connected to the output shaft 119 of the gear motor 91 by pin 121. Another gear 123 is mounted on the hub 106 and drives a large gear 124 mounted upon a hub 126 secured to the lower extremity of the outer shaft 74 by set screw 127.

In addition, the gear motor 91 is directly connected to a motor driven potentiometer 131 which gives an exact indication of the shaft position of the gear motor 91.

MONITOR SUBSTRATE SHIFTING ASSEMBLIES 22 AND 23 The monitor substrate shifting assembly 23 is shown in detail in'FIG. 4 of the drawings. As shown therein, it consists of a substrate 141 formed of a suitable material such as a glass or quartz disc and having a suitable diameter such as 8 inches. The substrate 141 is carriedby a hub assembly 142 mounted upon a shaft 143. The shaft 143 is rotatably mounted in a bearing housing 144 by a pair of ball bearing assemblies 146. The housing 144 is mounted upon the top wall 14 of the chamber 11 and secured tatably mounted in bearing assemblies 101 which are vheld in place by retaining rings 102. The bearing assemblies 101 are mounted in a cylindrical extension 61a formed as an integral part of the housing 61. A sun gear 103 is mounted upon the coupling 98 and drives a planetary cluster gear 104 mounted upon a hub 106. The hub 106 is secured to a shaft 107 by a pin 108. The shaft 107 is rotatably mounted in a pair of ball bearing assemblies 109 which are carried by a planetary arm 111 and retained therein by rings 112. The planetary arm 111;.is mounted upon a large hub 113 which is rotatably mountedby ball bearing assemblies 114 on the outer surface of the cylinr drical extension 614 and which a e reta thereon y thereto by suitable means such as cap screws 147.

The hub assembly 142 consists of an upper hub member 148 which is slidably mounted on thelower extremity of the shaft and is normally retained thereon by a retaining ring 149 mounted on the shaft 143. A spring 151 is disposed ,on the shaft 143 and has its lower extremity engaging the upper portion of the upper hub member 148 and has its upper extremity engaging a retaining ring 152 secured to the shaft 143. The hub assembly 142 also includes a lower hub member 154 which is secured to the lower end of the shaft 143 by a pin 156. The lower hub member 154 engages the lower surface .of the substrate 141 and is provided with a pin 157 which is disposed in a hole 158 provided in the substrate 141 so that when the shaft 143 is rotated, the substrate 141 is driven thereby. In addition, the spring 151 yieldably urges the upper hub member 148 into engagement with the upper surface of the substrate 141' so that the substrate 141 is firmly" clamped between the upper hub memberv and the lower hub member 154.

The shaft 143 is driven by a speed reducer 161 which is mounted in a speed reducer housing 162. The housing 162 is mounted upon the bearing housing 144 by a plate 163 which is secured to thebearing housing 144 by screws 164 andto the speed reducer housing 162 by screws 166. .The speed reducer 161 is providedwith an output shaft 167 which is disposed in a bore 168 provided inthe shaft 143 and is pinned to the shaft 143 by a pin 169L'The speed-reducer 161 is driven-by an electricgear motor .171 which issecuredjto the speed reducer housing 162 by screws 1 72. The motor is provided .with an output shaft 173 WhiCh is connected by a coupling 174 to. an input shaft 176 of the speed reducer 161, The gear motor 171 can be of any suitable type as, for example, it canbe one which has an output shaft which rotates at a. speed of '60 r.p.m.

The speedreducer, 161 can. still further reduce the speed so that the speed of rotation of the output shaft 167 .is approximately one half revolution per hour.

,Means is provided so that only a predetermined portion of the substrate 141 is exposed at any one time and consists, of a mainf'circular cover plate 181 secured to an annular bracket 182 bybolt's 183; the bracket 182 is secured.

to the bottom surfaces of the topwall 14 of the chamber 11 by screws 184. The cover plate 181 is provided with an arcuate opening or aperture 186 which subtends a suitable 181 which is inclined in the direction in which the vapors angle as, for example, an angle of The opening 186 is alsoof a suitable width as for example, onev inch. As can be seen from FIG.'.4, the opening 186 is defined on one.

side by an inclined surface 187 provided in the cover plate pass from the coating sources 16 and 17. A centrally disposed mounting member 188 is mounted pon the cover plate 181 and carries means which makes it possible to close off any portion or all of the opening 186. Such means consists of a segment or mask 191 which preferably sub.- tends an angle which is greater than the angle subtended by the opening 186 so that the opening or aperture canbe closed. The segment 191 is mounted upon a hub 192. The hub 192 is secured by suitable means such as a screw 193 and a washer 194 to the mounting member 188. It can be seen thatmerely by loosening the screw 193, the segment or mask 191 can be shifted to any desired angular position to close off any desired portion of the opening or aperture 186. 1

A plate 196 is secured to the cover plate 181 by suitable means such as welding and depends therefrom in a vertical direction. As can be seen. from FIG. 4, it is mounted on the cover plate 181 between the opening 186 and the source which is farthest away so that the sub.- strate 141 will only receive vapors from theclosest source and not from theother source, as illustrated in FIG. 1.

ELECTRICAL CIRCUITRY Suitable electrical circuitry is provided as a part of the coating apparatus and includes electronics shown in block diagram form in FIG. 6 for the mask indexing assembly and electronics shown in block diagram form in 'FIG. 7 for the automatic rate control system. The electronics which is shown in FIG. 6 forms a part of the coating apparatus and is provided to form a system which automatically programs the mixture proportion of two coating materials from the sources 16 and 17 by controlling the maskindexing assembly 21.

Electronics of the type shown in FIG. 6 is provided for the stepping motor 91 of the mask indexing assembly 21. As shown in FIG. 6, the electronics includes a curve tracer 201 of a suitable type such as manufactured by the F. L. Moseley Division of Hewlett-Packard Company. As is well known to those skilled in the art, the curve tracer is adapted to receive a sheet of paper upon which there is deposited a conducting ink which represents the proportionality relationship for the coating which is to be deposited upon the substrates 18 carried by the rack assembly 19. The curve tracer 201 produces a'signal proportional to the shape of the curve and supplies it to a differential amplifier 202. The signal form the curve tracer 201 serves as a reference signal and this signal is compared with a signal coming from the shaft position potentiometer 131 assuming that it is the stepping motor 91 which is being controlled by the electronics shown in FIG. 6. The differential amplifier takes the difference between the reference signal and the signal from the shaft position potentiometer 131 and supplies what is called an error output signal into two Schmitt trigger circuits 203 and 204. If the error is a positive error, the Schmitt trigger circuit 203 is operated, whereas if the error is negative, the otherSchmitt trigger cit cuit 204 is operated. If the error is negative; or, in other words, on one side of zero or one polarity, a relay 204 is operated by the trigger circuit 204 which is connected to the motor 91 to reverse the direction of rotation of the stepping motor 91.

If either a negative or a positive error signal is supplied by the differential amplifier to the Schmitt trigger circuits 203 and 204, a bistable flip-flop 207 is triggered. The bistable flip-flop 207 is of a type which supplies a squarewave to a power amplifier 208. The bistable flipflop 207 is also a type which, once triggered, will freerun until the errorsignal is zero. The stepping motor 91 is of a suitable type such as Slo-Syn 400 which is stepped by the squarewave from the power amplifier 208. Stepping of the motor 91 causes the mask 71 carried thereby to be shifted and at the same time causes the potentiometer 131 to be rotated a proportional amount until a signal is fed back to the differential amplifier 202 to reduce the error signal from the amplifier 202 to zero. As soon as there is no error, the Schmitt trigger circuits 203 and 6 204 are triggered to turn off the bistable flip-flop 207 which stops free running and which, in turn, causes the stepping motor 91 to stop rotating. As can be seen from =FIG.-6, stable voltage supplies 211 and 212 are provided for the curve tracer 201 and for the potentiometer 131, respectively.

Motor 92 is a non-programmed constant speed motor which operates directly on v. AC line voltage.

By utilizing this type of automatic control, it can be seen that the index of refraction of the material being deposited on the substrate can be varied from the index of refraction of one material being evaporated in the chamber 11 to the index of refraction of the other material being evaporated in the chamber 11. -In addition, it is possible to make a smooth transition between the indices of .refraction of the two materials being utilized. The automatic rate control system which is shownin FIG. 7 is, provided for controlling the rate of evaporation from each of the first and second source assemblies 16 and 17. In the. coating apparatus herein described, the automatic rate control system is utilized for maintaining a constant rate of evaporation from each of the coating sources 16 and 17. However, it should be appreciated that this automatic rate control system can be utilized for providing a rate of evaporation following any preprogrammed function. When a constant coating rate is desired, a constant reference is supplied to the reference terminal 217 in FIG. 7. If a pre-programmed function is to be followed, the function may be applied to a chart mounted on a curve tracer. The output from the curve tracer is supplied to the reference terminal 217. The rate of evaporation from the boat 36 is monitored by the evaporation rate monitor 27 and its transducer 28 which supplies a signal to an amplifier 218. The amplifier 218 can be of any suitable type such as the Type 415D manufactured by Hewlett-Packard Company.

The output from the amplifier 218 is supplied to a differential amplifier 219 which compares this feedback signal with the reference signal supplied to the differential amplifier from the reference terminal 217. The error signal which is supplied by the differential amplifier is measured by the error meter M1. The error signal is fed into a first compensator 221 through a non linear amplifier 220 and then to a second compensator 222. The compensators 221 and 222 prevent the system from hunting beyond desired ranges or, in other words, to prevent the system from becoming unstable. The first compensator generates a phase lead which is utilized for compensating for the dead time in the evaporation rate monitor. This dead time occurs because it takes a predetermined amount of time for the monitor disc 141 to rotate past the window or opening 186 provided in the main cover 181. The amount of this dead time changes as the window size is changed. The second compensator 222 generates a phase lead with a phase lag and compensates for a long time delay constant which is caused by the time required to change the temperature of the boat to change the evaporation rate.

The output from the second compensator 222 is supplied to a double Schmitt trigger and relay 223; similar to that shown in FIG. 6, which serves as a relay with dead time. The double Schmitt trigger and relay 223 supplies an output to a motor-driven Variac 226 which supplies either a bucking or a boosting voltage to the primary of a step-down transformer 227. The step-down transformer 227, in turn, supplies a bucking or boosting voltage to a boat transformer 228 in series with the voltage supplied by a manually controlled Variac 229. The boat transformer 228 then supplies a voltage to the boat 36 to heat the boat. Heating the boat causes material in the boat to evaporate to cause a vapor stream to flow therefrom and to be deposited upon the monitor substrate 141. The ratethe material is deposited upon the substrate 141 is monitored by the evaporation rate monitor 27 which supplies a signal to the amplifier 218 to provide the feedback signal for the differential amplifier 219 as hereinbefore described. When the error signal from the differential amplifier 219 reaches zero, the double Schmitt trigger circuit 223 will be burned off to stop the motor driven Variac 226.

It will be noted that a number of the blocks shown in FIG. 7 have been provided with conventional feedback control notation to indicate the manner in which they are designed.

Operation of the coating apparatus and system in performing the method can now be described briefly in conjunction with the rate monitor. Let it be assumed that the rack assembly 19 has been loaded with substrates 18 to be coated and that the desired materials to be utilized for the coating operation have been placed in the boats 36. The coating apparatus can then be placed in operation. The rack assembly 19 is continuously rotated and the mask indexing assembly 21 is placed in operation. Then, either one or both the source assemblies 16 and 17 are also placed in operation to heat the boats 36. It should be appreciated, however, that the mask indexing assembly 21 is under the control of electronics of the type shown inFIG. 6, whereas the source assemblies 16 and 17 are under the control of electronics such as that shown'in FIG. 7.

As can be seen from FIG. 1, while the coating apparatus is in operation, the masks 71 and 72 are positioned so that they control the flow of the vapor stream from each of the sources 36 to the substrates 18 carried by the rack assembly 19. Also, it can be seen from :FIG. 1 that each of the monitor substrate shifting assemblies will only receive a vapor stream from one of the two sources contained in the coating apparatus. This is because the member 196 prevents the vapor stream from the other of the sources from reaching the opening or window 186 provided in the cover plate 181. It also should be 'notedthat the monitor substrate shifting assemblies 22 and 23 actually see the vapor stream from their respective sources without their being chopped by the masks 71 and 72. In other words, they see their respective vapor sources continuously. By controlling the source assemblies 16 and 17 so they supply vapors at a constant rate to the monitor substrate shifting assemblies, it can be seen that by changing the phase relationship of the masks 71 and 72 under the, control of the electronics shown in 'FIG. 6, it is possible to vvary the rate of deposition of each of ,-the coating materials upon the substrates 18. With the arrangement shown, it can be seen that coatings of; the two different types of material can be deposited upon the substrates separately, or they can be deposited simultaneously to obtain any desired mixture. Thus,-one layer upon the substrate can consist of 30% ofone coating material, and 70% of the other material. Also, with the coating apparatus shown, it is possible toma'kea continuous transition from one index of refraction for one coating material to the other index of refraction. for-the other coating material. Also, it is possible to utilize the apparatus 'for providing a layer which has an index of refraction which is between the ones available from the two materials being utilized. Thus, it is possible byevaporating both materials simultaneously to obtain a layer having an index of refraction which has the same ratio as the mixture of the coating materials. For example, a layer comprised of 50% of one material and.50,% of the other material should be given an index of refraction 1 which is between the indices of refraction of, the two materials. In this way, it is possible to make coatings having indices of refraction which are notavailable. from materials which appear in nature. I

During the entire coating operation, the monitor-substrates 141 are rotating very slowly. For example, in one embodiment of the invention, the glass disc rotated has a speed of approximately one-half revolution perhour. With this s eed of rotation, it was found that each point of the substrate 141 was exposed to-the vapor stream for 3% minutes. By measuring the rate of deposition at the end of'the opening 186, it is possible to measure the average rate with the 3 /3 minute time constant. This evaporation rate is being monitored by the evaporation rate monitor '27. If the evaporation rate changes, the thickness of the layer on the substrate 141'changes which, in turn, changes the reflectance which is measured by the monitor 27 to give a direct indication of the rate of evaporation of the coating vapors.

The size of the opening 186 determines the thickness of the layer which is measured by the monitor 27. There are a numberof conflicting considerations involved. On the one hand, it is desirable to measure the reflectivity as quicklyas possible in orderto obtain a system which has a-fast reaction. On the other hand, it-is desirable to Wait a period of time to make the measurements so that the coating will havea thickness which Will give a large reflectivity which can be easily and accurately measured; The size of the opening of the hole 186 is, therefore, chosen so that it is a compromise between these two conflicting considerations. Thus, for a disc having a diameter of approximately 8 inches, an opening of inch hasbeen found to be satisfactory.

In addition, the opening 186 should be positioned so that it can see the source of the coating vapors at all times and will not at any time be obstructed by masks 71 and 72 In view of the fact that adhesion need not be-considered, the angle of incidence of the vapor stream on the substrate 141 is not particularly critical.

The substrate 141 is rotated at a relatively slow speed so that inone revolution of the same, one complete coating comprised of many layers can be completed. It can be readily appreciated that if longer coating periods are required, larger glass discs can be provided for the substrates 141 to give the additional required time. Alternatively, the disc or substrate 141 can-be rotated at a still slower rate.

The optical rate monitoring systemutilized in the coating apparatus has many advantages. It is relatively stable and .gives valarger signal than conventional devices. In addition, it-is very rugged and reliable. The rate monitor herein disclosed may be useful in other applications. For example, it may be useful for a continuous process in which-it is desired to deposit a coating of uniform thick nessthroughout-the process.

:It is apparent from the foregoing that we have provided a coating apparatus, system and method, and rate monitor therefor in which one or more evaporation sources may b'e-utilized independently of each other or at the-same time/ The rate of' evaporation from any of the 'sourcescan' be controlled automatically and can be made'to follow a pre-programmedfunc'tion. In addition, the rates Iof deposition of the materials upon thesubvstrates to be coated can be readily controlled by the use of.the maskingass'embly. At the same time, the rate of evaporation can be readily monitored to give the desired control.

We claim: i

1. In 'a method for/controlling theindex of refraction of. a layer upon aplurality of substrates by utilizing two coating materials having different indices of refraction from first and second sources, continuously causing relative movement'between the sources and the plurality'of substrates, evaporating the two coating materials simultaneously to produce first and second vapor streams which impinge upon the substrates,-monitoring the rates of evaporationof the two coating materials from the sources to supply monitoring information, automatically controlling the rates of evaporation from said sources by using said monitoring information to cause substantially constant evaporation rates of the two coating materials masking the substrates from thefirst and second vaporstreams by 9 the use of first and second masks, and automatically controlling the phase relationships between the first and second masks so that the coating materials from the first and second sources are deposited upon the substrates in a predetermined ratio.

2. A method as in claim 1 wherein said masking step is carried out with the first and' second masks located in closer proximity to the sources of the coating materials than to the substrates being coated and wherein said first and second masks are rotated in accordance with a preprogrammed function in cooperation with each other whereby the layer deposited upon the substrates is comprised of a mixture of the two coating materials with proportions related to the pre-programmed function.

3. A method as in claim 2 wherein the monitoring is carried out by independently monitoring the rate of evaporation from each of the first and second sources.

References Cited UNITED STATES PATENTS 2,160,981 6/1939 OBrien 11849 X 3,271,179 9/1966 Smith, Jr. 117106 R 3,063,867 11/1962 Emery, Jr 117106 R 3,316,386 4/1967 Yafie et a1. 118-49 3,071,533 1/ 1963 Blankenship 118-7 3,313,914 4/1967 Roberts, IL, et al 1187 3,347,701 10/1967 Yamagishi, et a1. 117106 R FOREIGN PATENTS 1,314,569 12/1962 France 350-166 1,051,402 12/ 1966 Great Britain 11849.5

RALPH S. KENDALL, Primary Examiner M. G. WITYSHYN, Assistant Examiner 

